Controlling the Thermoelectric Properties of Nb-Doped TiO₂ Ceramics through Engineering Defect Structures

Abstract

Donor-doped TiO 2 ceramics are promising high-temperature oxide thermoelectrics. Highly dense (1 − x)TiO 2 −xNb 2 O 5 (0.005 ≤ x ≤ 0.06) ceramics were prepared by a single-step, mixed-oxide route under reducing conditions. The microstructures contained polygonal-shaped grains with uniform grain size distributions. Subgrain structures were formed in samples with low Nb contents by the interlacing of rutile and higher-order Magneĺi phases, reflecting the high density of shear planes and oxygen vacancies. Samples prepared with a higher Nb content showed no subgrain structures but high densities of planar defects and lower concentrations of oxygen vacancies. Through optimizing the concentration of point defects and line defects, the carrier concentration and electrical conductivity were enhanced, yielding a much improved power factor of 5.3 × 10 −4 W m −1 K −2 at 823 K; lattice thermal conductivity was significantly reduced by enhanced phonon scattering. A low, temperature-stable thermal conductivity of 2.6 W m −1 K −1 was achieved, leading to a ZT value of 0.17 at 873 K for compositions with x = 0.06, the highest ZT value reported for single Nb-doped TiO 2 ceramics without the use of spark plasma sintering (SPS). We demonstrate the control of the thermoelectric properties of Nbdoped TiO 2 ceramics through the development of balanced defect structures, which could guide the development of future oxide thermoelectric materials.

High Power Factor Nb-Doped TiO2 Thermoelectric Thick Films: Toward Atomic Scale Defect Engineering of Crystallographic Shear Structures

“…The XPS data confirm that TF6G films contain a higher level of oxygen vacancies than TF6 films, contributing to the enhancement of n and σ. It is worth noting that the σ value for TF6G films at 673 K is approximately the same as that for densified bulk samples with the same composition, and this high σ value is still achieved with films containing approximately 17% porosity.…”

“… Therefore, the higher Nb doping of TF6 films gives rise to more carriers, and thus, the sharp decrease of S can be attributed to variations in n . However, the absolute S values of TF6 films are slightly higher than the previously reported values for similar compositions; this can be ascribed to the presence of significant amounts of TBs and different processing atmospheres. The higher density of TBs contributes to S by energy filtering effects; while the less reducing atmosphere results in lower oxygen deficiency, which reduces n and thereby enhances S .…”

“…Oxide ceramics are attracting increasing interest for TE applications as they are predominantly based on earth-abundant, environmentally benign elements and are stable over a wide range of temperatures. The families of materials include, but are not limited to, p-type cobaltites (NaCo 2 O 4 , Ca 3 Co 4 O 9 , Ca x CoO 2 , Ca 3 Co 2 O 6 , Co 3 O 4 , and Bi 2 Sr 2 Co 2 O y ), n-type strontium titanate (SrTiO 3 ), titania (TiO 1.1 , TiO 2– x , and TiO 2 ), zinc oxide (ZnO), CaMnO 3 , Magnéli phases (Ti n O 2 n –1 , Nb 12 O 29 , and WO 3– x ), indium oxide (In 2 O 3 ), and tungsten bronze-structured oxides (Ba 5.19 Nd 8.54 Ti 18 O 54 and Ba 6 Ti 2 Nb 8 O 30 ). Among these, TiO 2 -based oxides offer excellent thermal stability but so far modest TE performance.…”

“…However, a variety of strategies are availed for improving their TE transport properties. For instance, donor-doping of TiO 2 (by Nb, , Al, Ta, etc.) enhances electrical conductivity and carrier concentration and reduces thermal conductivity.…”

Phase transformation and interface crystallography between TiO2 and different TinO2n−1 phases

“…TiO 2 and related non-stoichiometric Ti n O 2 n −1 phases have been widely utilized in the fields of energy storage and conversion, 13–16 photocatalysis, 17–19 optoelectronic devices, 20,21 thermoelectricity, 22,23 etc. By employing different types of reducing agents, TiO 2 will gradually transform to the Ti n O 2 n −1 phase with different reduction degrees through a series of phase transitions.…”